Lithium salt/carbonate electrolyte system, a method for the preparation thereof, the use thereof and a battery containing the electrolyte system

ABSTRACT

The instant invention is to an electrolyte consisting essentially of a salt mixture and a solvent mixture. The solvent mixture consists essentially of ethylene carbonate and dimethyl carbonate and the salt mixture consists of 60-90% lithium tetrafluoroborate and 10-40% lithium hexafluorophosphate. A battery comprising this electrolyte and a method of preparing this electrolyte are exhibited.

This application is a 371 of PCT/DK97/00415, filed Sep. 30, 1997.

This invention relates to an electrolyte system, a method for thepreparation thereof, the use thereof, and a battery containing theelectrolyte system, and particularly to an electrolyte composition and arechargeable battery of high compatiblity towards positive electrodestructures, high cyclability and low irreversible loss.

The use of non-aqueous electrolytes has allowed the development ofelectrochemical systems based on lithium-containing negative electrodestructures and positive intercalation metal oxides, which have highenergy density. For those systems, however, the limiting factor fortheir continuous performance has been their low electrolyteelectrochemical stability, leading to poor cell cyclability.

Said cyclability is defined as the number of times a battery can becharged and discharged between 4.2V and 3.0V at a current density of 1mA/cm² before the capacity is reduced to 80% of the discharge capacityof the first discharge of the battery.

Upon operation of such electrochemical cells, a capacity loss duringinitial charging of the cells is observed, as is a fading capacity uponextended cycling. Those capacity reduction phenomena can be ascribed tothe instability of the electrolyte towards the negative and the positiveelectrode of the electrochemical cell. The instability towards thenegative electrode leads to gassing and formation of a passivating film.The instability towards the positive electrode leads to corrosion of theelectrode structure. Both phenomena result in loss of active materialfrom the cell. Further, passivation and corrosion may lead to increasedcell impedance, and reduced materials utilisation and rate capability.

The gassing and the formation of the passivating film is believed totake place during the initial charging, subsidiary during the young lifeof the electrochemical cell. Therefore, the capacity loss during theinitial charging is mainly ascribed to the instability of theelectrolyte-negative electrode structure-system. The corrosion is anongoing process, which is believed mainly to take place towards the endof each charging cycle at maximum potential of the positive electrodestructure. Therefore, the capacity loss during cycling is mainlyascribed to the instability of the electrolyte-positive electrodestructure-system, i.e. lack of compatibility of the electrolyte towardsthe positive electrode structure.

The capacity loss during the first charging of the battery is referredto as the initial irreversible loss, and is defined as the loss ofactive lithium in mAh of charge vs. the weight of the negative electrodestructure in grains. In this context the loss is defined as activematerial which reacts irreversibly with the negative electrodestructure, and which cannot be deintercalated from the structure duringthe subsequent discharge of the battery.

The known electrolyte or electrolyte systems are mainly composed of oneor more salts in a solvent or solvent mixture. Traditionally, a numberof salts are commonly applied in non-aqueous electrolyte systems,including lithium perchlorate, lithium hexafluorarsenate, lithiumtrifluoromethylsulfonate, lithium tetrafluoroborate, lithiumhexafluorophosphate, lithium bromide, lithium hexafluoroantimonate,LiC(CF₃SO₂)₃ and LiN(CF₃SO₂)₂. The solvents used are e.g. ethylenecarbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate,γ-butyrolactone, γ-valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, diethoxyethane, methyl acetate, methylformate, 1.3-dioxolane, sulfolane, acetonitrile and butyronitrile.

A large number of electrolyte systems have been composed from thesesalts and solvents in order to improve inter alia the electrochemicalstability of the systems.

It has now been found that the cyclability and stability of anelectrochemical system such as a battery may be improved considerably,by using a electrolyte system having the composition of claim 1.

By using salt mixtures of lithium tetrafluoroborate and lithiumhexafluorophosphate, excessive in the borate, in batteries it has beenobserved, that high cyclability (low capacity loss during extendeddischarge-recharge) and a low initial irreversible loss can be obtained.

This excellent performance has been found for those salts compositionscontaining 60-90% by mole of lithium tetrafluoroborate and 10-40% bymole of lithium hexafluorophosphate. These salt compositions arebelieved to be dominated by a high compatibility of the borate againstthe positive electrode structure, supported by negative structurecompatibility and contribution to the conductivity from the phosphate.

It has further been found that the performance is especially pronouncedfor the high stability solvent mixture of ethylene carbonate anddimethyl carbonate and particularly if these solvents are the onlyliquid solvents present. Further, the performance of the electrolytesystem is especially pronounced, when combined with negative electrodestructures of carbonaceous intercalation materials like coke andgraphite and/or positive electrode structures of lithium manganese oxidespinels.

A number of patents describe the use of the lithiumtetrafluoroborate/lithium hexafluorophosphate salts in a solvent mixtureconsisting of or comprising ethylene carbonate and dimethyl carbonate.

U.S. Pat. No. 5,192,629 and U.S. Pat. No. 5,422,203 of BellCommunication Research describe the use of an electrolyte system with amixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) assolvent and lithium hexafluorophosphate and mixtures of lithiumhexafluorophosphate with up to about equal mole parts of lithiumtetrafluoroborate as solute. The teaching in these patents is that thisgroup of electrolytes provides an improved resistance against oxidation.The only example showing the effect of the electrolyte, is composed from1M LiPF₆ in 95 DMC:5 EC. Though it is mentioned that the electrolyte maycomprise lithium tetrafluoroborate, nothing is mentioned about theeffect of lithium tetrafluoroborate.

U.S. Pat. No. 5,079,109 of Toshiba Battery Co., Ltd. describes anelectrolytic salt consisting of one of lithium phosphate hexafluoride(LiPF₆) and lithium borofluoride (LiBF₄) in a solvent mixture consistingof ethylene carbonate, 2-methyltetrahydrofuran and at least oneester-based nonaqueous solvent selected from butylene carbonate,dimethyl carbonate, γ-butyrolactone and sulfolane. In none of thenumerous examples of this patent, however, the mixture of saidelectrolytic salts and the positive effect thereof are described.

U.S. Pat. No. 5,296,318 and U.S. Pat. No. 5,429,891 of BellCommunication Research, Inc. describe a solid electrolyte for arechargeable lithium intercalation battery cell of a self-supportingfilm of a copolymer of vinylidene fluoride and hexafluoropropylene, theelectrolyte thereof being selected from LiPF₆, LiAsF₆, LiCIO₄,LiN(CF₃SO₂)₂, LiBF₄, LiCF₃SO₃ and LiSbF₆ and the solvent thereof beingselected from ethylene carbonate, propylene carbonate, dimethylcarbonate, diethoxyethane, diethyl carbonate, dimethoxyethane, dipropylcarbonate and mixtures thereof. The patent, however, describes nomixtures of said electrolytic salts and their performance.

G. Pistoia, A. Antonini, R. Rosati and D. Zane (Electrochemica Acta 412683-9 (1996)) describe cathodes for Li-ion batteries based onelectrolyte compositions of either LiPF₆ or LiBF₄ in a mixture ofethylene carbonate and dimethyl carbonate. The authors do not describe,however, the salt mixtures of the present invention.

U.S. Pat. No. 5,370,949 of National Research Council of Canada describesa non-aqueous electrolyte of which the salt is selected from LiAsF₆,LiPF₆, LiBF₄, LiCIO₄, LiBr, LiAICI₄, LiCF₃SO₃. KLiC(CF₃SO₂)₃ andLiN(CF₃SO₂)₂ and mixtures thereof a solvent thereof being selected fromethylene carbonate, 2-methyl tetrahydrofuran, tetrahydrofuran,dimethoxyethane, diethoxyethane, dimethyl carbonate, diethyl carbonate,methyl acetate, methyl formate, γ-butyrolactone, 1,3-dioxolane,sulfolane, acetonitrile, butyronitrile, trimethylphosphite anddimethylformamide and mixtures thereof. The patent, however, does nodescribe the good performance of the specific mixture of lithiumhexafluorophosphate and lithium tetrafluoroborate in a mixture ofethylene carbonate and dimethyl carbonate.

WO 95/34920 of Lexcel Technology Ltd. describes the use of electrolytesof LiBF₄ and/or LiPF₆ and/or lithium trifluoromethane sulphonamide in amixture of a linear carbonate and a cyclic ester, where said linearcarbonate may be e.g. Diethyl carbonate or dimethyl carbonate, and saidcyclic ester may be e.g. polypropylene carbonate or ethylene carbonate.The patent, however, does no describe the mixture of lithiumhexafluorophosphate and lithium tetrafluoroborate in a mixture ofethylene carbonate and dimethyl carbonate according to the presentinvention and its performance.

In a preferred embodiment of the present invention the solvent mixturecomprises ethylene carbonate and dimethyl carbonate in a relative molarratio of from 80:20 to 20:80, more preferably in a relative molar ratioof from 80:20 to 50:50, even more preferably in a relative molar ratioof from 75:25 to 60:40.

In an alternative embodiment of the invention the solvent mixture inaddition to ethylene carbonate and dimethyl carbonate comprises one ormore of the following solvents:

(a) other alicyclic carbonates represented by the following generalformula:

—C(═O)—O—CR₁R₂—[CR₃R₄]_(m)—CR₅R₆—O—,

 wherein each of R₁, R₂, R₃, R₄, R₅ and R₆ independently representhydrogen or C₁-C₄ alkyl groups and in is an integer equal to 1 or 5,preferably propylene carbonate;

(b) aliphatic carbonates represented by the general formulaR₇[OC(O)]_(p)OR₈, wherein R₇ and R₈ independently represent C₁-C₄ alkylgroups, and p is an integer equal to 1 or 2, with the proviso that R₈represent C₂-C₄ alkyl groups, when R₇ is methyl, and p=1, preferablydiethyl carbonate;

(c) lactones in the form of cyclic esters represented by the generalformula:

—C(═O)—CR₉R₁₀—CR₁₁R₁₂—[CR₁₅R₁₆]_(r)—CR₁₃R₁₄—O—

 wherein each of R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, R₁₅ and R₁₆ independentlyrepresent hydrogen or C₁₋₂ alkyl groups and r is an integer equal to 0or 1, preferably γ-valerolactone or γ-butyrolactone;

(d) esters represented by the formula R₁₇[C(O)]OR₁₈[OR₁₉]_(t), whereineach of R₁₇, R₁₈ and R₁₉ independently represent hydrogen or C₁-C₂ alkylgroups, and t is an integer from 0 to 2, preferably an acetate, morepreferably (2-methoxyethyl)-acetate and/or ethyl acetate;

(e) glymes represented by the general formula R₂₀O(R₂₁O)_(n)R₂₂, inwhich R₂₀ and R₂₂ independently represent C₁₋₂ alkyl groups, R₂₁ is—(CR₂₃R₂₄CR₂₅R₂₆)— wherein R₂₃, R₂₄, R₂₅ and R₂₆ independently eachrepresent hydrogen or C₁-C₄ alkyl groups, and n is an integer from 2 to6, preferably 3, R₂₀ and R₂₂ preferably being methyl groups, R₂₃, R₂₄,R₂₅ and R₂₆ preferably being hydrogen or C₁-₂ alkyl groups, morepreferably hydrogen.

In a preferred embodiment of the invention the mixture of saltscomprises lithium tetrafluoroborate and lithium hexafluorophosphate in arelative molar ratio of from 85:15 to 70:30.

In a further preferred embodiment of the invention the lithiumtetrafluoroborate and lithium hexafluorophosphate are present in a totalconcentration in the range from 0.0M to 3M, preferably 0.1M to 2M, morepreferably 0.5M to 1.5M.

It is further preferred that the electrolyte system comprises acrosslinked or non-crosslinked, preferably non-crosslinked polymer,preferably a non-crosslinkable polyester, polyurethane, polyether,polyacrylate, polyvinylidene fluoride or polyolefin. Said polymer ismore preferably selected from polymerised mono-, di- and trifunctionaloligomeric acrylates, preferably polyurethane acrylates and polyetheracrylates, or from polymerised mono-, di- and trifunctional monomericacrylates, preferably monofunctional monomeric acrylates, morepreferably urethane acrylate.

Said polymer may be present in an amount corresponding to from 0% to 50%by weight, preferably 2 to 25% by weight relative to the total weight ofthe electrolyte system.

In an embodiment of the invention the electrolyte system is confined ina separator consisting of a porous structure made from a polymer,preferably polyethylene, polypropylene, polycarbonate, cellulose, glassfiber or any inorganic material. Said separator is preferably of a wovenor non-woven structure having a pore size in the range of 10×10 nm to1×1 mm. Further, the separator has a thickness of 10-100 μm, preferably10-25 μm.

The present invention relates also to a method for the preparation of anelectrolyte system as defined above comprising the steps of mixing thesolvents, dissolving the salts in the solvent mixture, optionally addinga thickening agent to the solution, optionally incorporating the mixtureobtained in a separator, and optionally adding monomers or oligomershaving one or more polymerisable functional groups and inducingpolymerisation of these monomers or oligomers. Finally the electrolytesystem may be sandwiched between electrode laminates of anelectrochemical system like a battery, capacitor or electrochromicdisplay.

The invention further relates to the use of the electrolyte system in abattery, which may be any lithium battery in which the negativeelectrode may consist essentially of graphites, cokes, or any otherintercalation compound like oxides of tin, titanium, vanadium, cobalt ormanganese, and in which the positive electrode structure may comprise anintercalation compound, preferably of the transitions metal oxide type,more preferably lithium manganese oxide, lithium cobalt oxide orvanadium oxides, even more preferably spinel lithium manganese oxide.

In a preferred embodiment of the invention the negative electrode in thebattery consists essentially of graphites and cokes, which are able tointercalate lithium and which have an interlayer distance d₀₀₂ of nomore than 3.48 Å.

The positive effect of the electrolyte compositions of the presentinvention are especially pronounced when combined with such negativeelectrode strictures of the graphite and coke type.

Surprisingly, the effect is mostly pronounced when the electrolytecompositions are combined with the graphite material having the tradename “SFG 15” from TIMCAL G+T (Lonza G+T, CH) or when they are combinedwith any coke material of an interlayer distance d₀₀₂ in the range3.42-3.44 Å.

The positive electrode of the battery is preferably based on lithiummanganese oxide spinel.

Alternatively, the electrolyte system may be used in a capacitor or inan electrochromic display.

EXAMPLES

The invention is further illustrated in the following examples.

In these examples the terms “cyclability” and “initial irreversibleloss” have the meaning defined above, unless otherwise specified,

Further, graphite refers to a synthetic graphite of an interlayer,distance d₀₀₂ of 3.36 and coke refers to a petroleum coke of aninterlayer distance d₀₀₂ of 3.46 Å, unless otherwise specified.

Lithium tetrafluoroborate, lithium hexafluorophosphate, ethylenecarbonate, dimethyl carbonate and propylene carbonate all refer tobattery grade compounds from Merck, whereas lithium manganese oxiderefers to a lithium manganese oxide spinel structure of chemicalcomposition LiMn₂O₄.

Example 1

A battery was prepared with an electrolyte composition of 70.30 by moleof ethylene carbonate and dimethyl carbonate and a 75:25 by mole saltmixture of lithium tetrafluoroborate and hexafluorophosphate of a totalsalt concentration of 1M. The electrolyte was prepared by simple mixing.The battery was prepared by sandwiching said electrolyte system betweena negative electrode structure of graphite and a positive electrodestructure of lithium manganese oxide. Both of the active electrodematerials were mixed with 5% of carbon black for improved electronicconductivity.

Upon cycling of the battery it displayed a cyclability of more than 350cycles. In terms of irreversible loss during the initial charging, thecell displayed a loss of 50 mAh/g of anode material.

Comparative Example 1a

For comparison, a battery was prepared with an electrolyte compositionsof 70:30 by mole of ethylene carbonate and dimethyl carbonate and a saltmixture of 25:75 by mole of lithium tetrafluoroborate andhexafluorophosphate of a total salt concentration of 1M. The electrolytewas prepared by simple mixing. The battery was assembled by sandwichingsaid electrolyte system between a negative electrode structure ofgraphite and a positive electrode structure of lithium manganese oxide.

Both of these active electrode materials were mixed with 5% of carbonblack for improved electronic conductivity.

Upon cycling of the battery it displayed a cyclability of 150 cycles,displaying an initial irreversible loss of 80 mAh/g of the anodematerial.

Comparative Example 1b

For further comparison, another battery was prepared with an electrolytecompositions of 70:30 by mole of ethylene carbonate and dimethylcarbonate and pure lithium tetrafluoroborate as electrolyte salt in aconcentration of 1M. The electrolyte was prepared by simple mixing. Thebattery was assembled by sandwiching said electrolyte system between anegative electrode structure of graphite and a positive electrodestructure of lithium manganese oxide. Both of these active electrodematerials were mixed with 5% of carbon black for improved electronicconductivity.

Upon cycling of the battery it displayed a cyclability of 300 cycles,however, displaying an initial irreversible loss of 100 mAh/g of theanode material.

Comparative Example 1c

For even further comparison, yet another battery was prepared applyingan electrolyte compositions of 70:30 by mole of ethylene carbonate anddimethyl carbonate and pure lithium hexafluorophosphate as electrolytesalt at a concentration of 1M. The electrolyte was prepared by simplemixing. The battery was assembled by sandwiching said electrolyte systembetween a negative electrode structure of graphite and a positiveelectrode structure of lithium manganese oxide. Both of these activeelectrode materials were mixed with 5% of carbon black for improvedelectronic conductivity.

Upon cycling of the battery it displayed a cyclability of only 100cycles, displaying an initial irreversible loss of 50 mAh/g of the anodematerial.

Comparative Example 1d

For further comparison, another battery was prepared applying anelectrolyte compositions of 70:30 by mole of ethylene carbonate andpropylene carbonate and a 75:25 by mole salt mixture of lithiumtetrafluoroborate and hexafluorophosphate of a total salt concentrationof 1M. The electrolyte was prepared by simple mixing. The battery wasassembled by sandwiching said electrolyte system between a negativeelectrode stricture of graphite and a positive electrode structure oflithium manganese oxide. Both of these active electrode materials weremixed with 5% of carbon black for improved electronic conductivity.

Upon cycling of the battery it displayed a cyclability of only 5 cycles,however, displaying a high initial irreversible loss of more than 500mAh/g of the anode material.

Example 2

A battery was prepared applying an electrolyte compositions of 80:20 bymole of ethylene carbonate and dimethyl carbonate and a salt mixture of70:30 by mole of lithium tetrafluoroborate and hexafluorophosphate of atotal salt concentration of 1M. The electrolyte was prepared by simplemixing. The battery was assembled by sandwiching said electrolyte systembetween a negative electrode structure of graphite SFG15 from TIMCAL anda positive electrode structure of lithium manganese oxide. Both of theseactive electrode materials were mixed with 5% of carbon black forimproved electronic conductivity.

Upon cycling of the battery it displayed a cyclability of more than 350cycles, displaying a high reversible capacity of 360 mAh/g of the anodeand an initial irreversible loss of less than 50 mAh/g of the anodematerial.

Example 3

Another battery was prepared applying an electrolyte compositions of80:20 by mole of ethylene carbonate and dimethyl carbonate and a saltmixture of 80:20 by mole of lithium tetrafluoroborate andhexafluorophosphate of a total salt concentration of 1M. The electrolytewas prepared by simple mixing. The battery was assembled by sandwichingsaid electrolyte system between a negative electrode structure of cokeand a positive electrode structure of lithium manganese oxide. Both ofthese active electrode materials were mixed with 5% of carbon black forimproved electronic conductivity.

Upon cycling of the battery it displayed a cyclability of more than 300cycles (in this case the battery was cycled between 4.2V and 2.5V),displaying an initial irreversible loss of less than 70 mAh/g of theanode material.

Example 4

A battery was prepared applying an electrolyte compositions of 60:30:10by mole of ethylene carbonate, dimethyl carbonate and propylenecarbonate and a salt mixture of 80:20 by mole of lithiumtetrafluoroborate and hexafluorophosphate of a total salt concentrationof 1M, The electrolyte was prepared by simple mixing. The battery wasassembled by sandwiching, said electrolyte system between a negativeelectrode structure of graphite and a positive electrode structure oflithium manganese oxide. Both of these active electrode materials weremixed With 5% of carbon black for improved electronic conductivity.

Upon cycling of the battery it displayed a cyclability of more than 300cycles, displaying an initial irreversible loss of less than 70 mAh/g ofthe anode material.

Example 5

A battery was prepared applying an electrolyte compositions of 80:20 bymole of ethylene carbonate and dimethyl carbonate and a salt mixture of80:20 by mole of lithium tetrafluoroborate and hexafluorophosphate of atotal salt concentration of 1M. The electrolyte was prepared by simplemixing. The electrolyte was incorporated in a microporouspolyethylene-based separator. The battery was assembled upon sandwichingsaid electrolyte system between a negative electrode structure ofgraphite and a positive electrode structure of lithium manganese oxide.

Both of these active electrode materials were mixed with 5% of carbonblack for improved electronic conductivity.

Upon cycling of the battery it displayed a cyclability of more than 250cycles, displaying an initial irreversible loss of less than 50 mAh/g ofthe anode material.

What is claimed is:
 1. An electrolyte system consisting essentially of:a solvent mixture, and a salt mixture, characterised in that the solventmixture essentially consists of ethylene carbonate and dimethylcarbonate, that the salt mixture consists of lithium tetrafluoroborateand lithium hexafluorophosphate, and that the salt mixture comprises60-90% by mole of lithium tetrafluoroborate and 10-40% by mole oflithium hexafluorophosphate.
 2. An electrolyte system according to claim1, in which the solvent mixture essentially consists of ethylenecarbonate and dimethyl carbonate in a relative molar ratio of from 80:20to 20:80.
 3. An electrolyte system according to claim 1, in which thesolvent mixture consists entirely of ethylene carbonate and dimethylcarbonate.
 4. An electrolyte system according to claim 1, wherein thesolvent mixture in addition to ethylene carbonate and dimethyl carbonatecomprises one or more of the following solvents: (a) other alicycliccarbonates represented by the following general formula:—C(═O)—O—CR1R2-[CR3R4]m-CR5R6—O—,  wherein each of R1, R2, R3, R4, R5and R6 independently represent hydrogen or C1-C4 alkyl groups and m isan integer equal to 1 or 5; (b) aliphatic carbonates represented by thegeneral formula R7[OC(O)]pOR8, wherein R7 and R8 independently representC2-C4 alkyl groups, and p is an integer equal to 1 or 2, with theproviso that R8 represent C2-C4 alkyl groups, when R7 is methyl, andp=1; (c) lactones in the form of cyclic esters represented by thegeneral formula: —C(═O)—CR9R10-CR11R12-[CR15R16]r-CR13R14—O—  whereineach of R9, R10, R11, R12, R13, R14, R15 and R16 independently representhydrogen or C1-2 alkyl groups and r is an integer equal to 0 or 1; (d)esters represented by the formula R17[C(O)]OR18[OR19]t, wherein each ofR17, R18 and R19 independently represent hydrogen or C1-C2 alkyl groups,and t is an integer from 0 to 2; (e) glymes represented by the generalformula R20O(R21O)nR22, in which R20 and R22 independently representC1-2 alkyl groups. R21 is —(CR23R24CR25R26)- wherein R23, R24, R25 andR26 independently each represent hydrogen or C1-C4 alkyl groups, and nis an integer from 2 to
 6. 5. A electrolyte system according to claim 1,in which the mixture of salts comprises lithium tetrafluoroborate andlithium hexafluorophosphate in a relative molar ratio of from 85:15 to70:30.
 6. An electrolyte system according to claim 1, in which lithiumtetrafluoroborate and lithium hexafluorophosphate are present in a totalconcentration in the range from 0.0M to 3M.
 7. An electrolyte systemaccording to claim 1, in which the electrolyte system comprises acrosslinked or non-crosslinked polymer.
 8. An electrolyte systemaccording to claim 7, wherein said polymer is a non-crosslinked polymer.9. An electrolyte system according to claim 7, wherein said polymer isone or more members selected from the group consisting of polymerizedmonofunctional oligomeric acrylates, polymerised difunctional oligomericacrylates and polymerised trifunctional oligomeric acrylates; or fromthe group consisting of polymerised monofunctional monomeric acrylatepolymerised difunctional monomeric acrylates and polymerisedtrifunctional monomeric acrylates.
 10. An electrolyte system accordingto claim 7, wherein said polymers present in an amount corresponding tofrom 0% to 50% by weight, relative to the total weight of theelectrolyte system.
 11. An electrolyte system according to claim 1, inwhich the electrolyte system is confined in a separator consisting of aporous structure made from a polymer.
 12. An electrolyte systemaccording to claim 11 in which said separator is a woven or non-wovenstructure having a pore size in the range of 10×10 nm to 1×1 mm.
 13. Anelectrolyte system according to the claim 11, in which said separatorhas a thickness of 10-100 μm.
 14. A method for the preparation of anelectrolyte system according to each of the preceding claims, comprisingthe steps of: mixing the solvents, dissolving the salts in the solventmixture at a molar ratio of 60%-90% LiBF₄ to 10%-40% LiPF₆, optionallyadding a thickening agent to the solution, optionally incorporating themixture obtained in a separator, and optionally adding monomers oroligomers having one or more polymerisable functional groups andinducing polymerisation of these monomers or oligomers.
 15. A batterycomprising a negative electrode, a positive electrode, and anelectrolyte system, characterized in that the electrolyte system is anelectrolyte system according to claim
 1. 16. A battery according toclaim 15, characterised in that the negative electrode consistsessentially of graphites, cokes, oxides of tin, oxides of titanium,oxides of vanadium, oxides of cobalt or oxides of manganese.
 17. Abattery according to claim 16, characterised in that the negativeelectrode consists essentially of graphites and cokes of an interlayerdistance d₀₀₂ of not more than 3.48 Å.
 18. A battery according to claim17, characterised in that the negative electrode consists essentially ofone or more graphites.
 19. A battery according to claim 17,characterized in that the negative electrode consists essentially of acoke of an interlayer distance d₀₀₂ in the range 3.42-3.44 Å.
 20. Abattery according to claim 15, characterised in that the positiveelectrode structure comprises an intercalation compound.
 21. A batteryaccording to claim 20, characterised in that the positive electrodestructure consists essentially of spinel lithium manganese oxide.
 22. Abattery comprising the electrolyte system of claim 1.